Phase shift mask technology approaches have been discussed thoroughly in the literature; the original work was begun by M. D. Levenson et al, "Improving Resolution in Photolithography with a Phase-Shifting Mask", IEEE Transactions on Electron Devices, Vol. ED-29, No. 12, pp. 1828-1836 (Deco 1982).
Attenuated phase-shift masks (APSMs) have been disclosed that employ a thin layer which is partially light transmissive and is involved in the phase shift. One such approach uses a thin layer of chromium (a few hundred angstroms, .ANG.), while a second such approach uses a chromium oxide. In the first approach, 30 nm of Cr and dry-etching into quartz 0.42 .mu.m deep is used to achieve 180.degree. phase shift. In the second approach, a thicker layer of a chromium oxide, on the order of 200 nm, and an isotropic etch into the quartz substrate, 0.04 .mu.m deep, is used to achiever 180.degree. phase shifts The foregoing values are based on using i-line wavelength (365 nm).
Simulation results using a vector simulator called TEMPEST (time-domain electromagnetic massively parallel evaluation of scattering on topography) show that the first approach gives better results, compared to the second approach. Exposure-defocus graphs, for example, show that for chromium oxide masks, focus budget is significantly less than for thin chrome ones. Attenuated phase-shift masks print smaller critical dimensions (CDs). The correlation between simulation and experimentation has been established; see, e.g., R. A. Ferguson et al, "Impact of Attenuated Mask Topography on Lithographic Performance", SPIE Meeting, March 1994, paper no. 2197-12.
The drawback with both of the foregoing approaches is that both require etching into the silicon dioxide (quartz) substrate to obtain the correct phase shift of 180.degree.. Such etching is difficult to control within very tight specification tolerances. Thus, what is needed is a process that provides the correct phase shift of 180.degree. without etching into the silicon dioxide substrate.